The assay of turbidity is an effective means for evaluating the level of contaminates and pathogens within a liquid stream. The determination of a constituent in a liquid flowing in a stream may be carried out using a variety of methods and techniques. A significant number of methods and analysis techniques currently rely on interrogating a liquid sample using optical means. The interrogation includes a beam of light, or other electromagnetic radiation, being transmitted through the liquid. The light will be absorbed, scattered, or stimulate fluorescence in proportion to a determinable component of the liquid, analyte, or contaminate of interest.
Entrained air and other gases present within the liquid sample can cause a portion of the beam of light to scatter as the light travels through the liquid sample resulting in a reduction in the transmission of light through the liquid and an increase of the observability of the beam within the liquid. The decrease in the transmission of light through the sample due to entrained air mimics absorption and is indistinguishable from that which is due to the analyte, contaminate, or pathogen. The observability of the beam due to entrained air likewise interferes with nephelometric or fluorometric determination methods as more light is scattered than can be accounted for due to particle content or fluorescence of the liquid sample.
It is therefore important for an accurate determination of the constituent of interest that air or other entrained gases be removed prior to interrogation of the liquid sample by a beam of light to the extent that the remaining air or gases have no significant contribution to the limit of detection of the method of analysis or is reduced to less than that which does not interfere with the determinable property of the liquid.
When the removal of entrained gases is incomplete due to changes in the measurement conditions (e.g., pressure, temperature, or flow rate of the liquid sample stream) other or additional means must be employed for removal of the interference from affecting an integrity of the value of interest. Other or additional means for removal of the fine bubble interference value from the turbidity value may be employed during the interrogation or determination steps. To this end, bubble rejection algorithms are commonly employed to diminish the error introduced by entrained gas within the assay process by eliminating statistical outliers from a measurement data series. At lower flow rates, the interference value due to fine gas bubbles is readily distinguishable from the turbidity value as the summary product of the interference value and the turbidity value. For instance, a baseline signal value is substantially equal to the turbidity value.
A signal value where the interference value is significantly higher than the baseline value may be readily removed from a given measurement set during the determination step as a statistical outlier. To disadvantage, at low flow rates the observability of fluctuations in the analyte concentration within an assay chamber is diminished due to a low exchange rate of the incoming liquid sample mixing with the existing liquid sample within a volume of the assay chamber.
Another consequence of too low a flow rate is an increase in the delay from when a change in concentration event occurs to when the change in concentration is observed. Of further consequence of flow rate on the assay determinability, as the liquid sample flow rate is increased, a greater number of fine gas bubbles are more often carried into the assay chamber of the analytical device making the baseline determination less obvious.
As the flow rate increases further, the frequency at which the interference value is superimposed upon the turbidity value continues to increase until a limit is reached where the baseline value becomes irreconcilable from the interference value. It is therefore necessary to reduce the flow rate or alter the measurement step(s) and/or determination step(s) so as the resulting baseline value is no longer obscured by the fine bubble interference. For instance, a flow rate where the interference rate does not exceed the Nyquist limit of the interrogation or measurement rate. Stated alternatively, in practice it is required that the liquid assay device, (e.g., the turbidimeter, nephelometer, or alike), be operated at a flow rate where the frequency at which interference value due to fine bubbles imprinted on the assay value does not exceed one half the assay measurement rate.
A further problem exists in the determination of flow rate at flows of less than 100 mL per minute, where the head pressure to the flow meter is low and the liquid sample is sedimentary in nature. Conventional flow meters such as paddle-wheel (turbine), differential pressure, variable area, caloric, positive displacement, Coriolis, weir height (open channel), and vortex type flow meters have a disadvantage of high cost, high power consumption, and/or are unreliable in conditions of low flow rates. They further require high head pressure and clog easily when used for measuring liquid samples containing particulate matter.